These plots, all foiled, were discovered to be the work of extremists, based in Yemen, linked with al Qaeda.

Author and Yemen scholar Gregory D. Johnsen has written a comprehensive history of al Qaeda in Yemen, and shares his thoughts with The World's Marco Werman. He observes that al Qaeda's roots in Yemen go back to the 1990s.

Americans may first have learned of al Qaeda in October, 2000, when the group detonated a bomb that blew a hole in the side of the USS Cole. This was not the group's first attempt to hit a US warship. Says Johnsen, "al Qaeda had attempted to carry out a similar attack in January of 2000, and they overloaded the dinghy that they were going to use to attack a US war ship, and it sunk."

After 2001, al Qaeda members in Yemen were arrested and imprisoned. But when many escaped in a 2006 prison break, authorities paid little attention. Says Johnsen, "al Qaeda essentially had two years in which to rebuild their organization with no sort of interference from the US or Yemeni governments. And the result was the embassy attack in 2008, and all the attacks that have taken place since then."

Johnsen says the US wants to avoid becoming mired in a Yemen ground war similar to the one the US has fought in Afghanistan. Rather, it is targeting top al Qaeda leaders in Yemen with air strikes, drone attacks and covert operations.

"The problem is that the US says there are 10 to 15 al Qaeda commanders that it's trying to kill in Yemen," says Johnsen. "This year it's carried out anywhere from 37 to 50 strikes in Yemen — in an attempt to kill 10 to 15 individuals. So, in my view, one of two things is happening. Either the drone strikes are not as accurate as we are continually being told that they are, or that the US is targeting many more individuals than those 10 to 15 on its list. And I think that if it's the latter then the US really does run the risk of being sucked into a much longer, much costlier conflict in Yemen."

Gregory D. Johnsen's book is entitled The Last Refuge: Yemen, Al Qaeda, and America's War in Arabia.